This invention relates to a continuous vacuum pan (also sometimes referred to as an evaporative crystallizer) for use in the sugar processing industry, and to an insulation arrangement used inside the continuous vacuum pan. More particularly, but not exclusively, the invention relates to an insulating arrangement between a calandria and a down-take of the continuous vacuum pan.
In this specification the term “calandria” shall be interpreted to mean a shell and tube reboiler commonly used in continuous pans found in the sugar processing industry. The calandria may generally be of a floating or a fixed configuration, as is known in the art.
During one particular stage of the sugar production process, syrup produced by evaporators is concentrated further in specially designed vessels known as pans. As the concentration rises the dissolved sugar crystallises and the work of the pans is to grow sugar crystals (from the sucrose in syrup) in several steps to maximise the amount of sucrose recovered in raw sugar. It will be appreciated that this is a crucial step in the sugar production process, and hence much attention has been given to the design of pans, and in particular continuous vacuum pans, in recent times.
A vacuum pan is essentially a vessel, operated under vacuum, in which sugar syrup is boiled in order to increase the sugar concentration, and thus resulting in the formation of sugar crystals, resulting in a suspension of crystals within the mother liquor from which they are growing (so-called massecuite). A calandria is generally used as a reboiler to heat the massecuite and also to cause circulation of massecuite inside the vessel. Steam is supplied to the calandria via a steam inlet, and is conveyed between the tubes of the calandria, condensing on the tube walls, thus resulting in effective heat transfer from the calandria to the massecuite. One particular continuous vacuum pan design is disclosed in the applicant's own prior patent, U.S. Pat. No. 6,991,708, the contents of which is incorporated herein by reference.
A critically important aspect of vacuum pan design is ensuring that there is good circulation of massecuite within the pan. The desired circulation path starts with massecuite flow upwards through the tubes of the calandria (that are heated on their outsides) and then on exiting the tubes, flows over the tops of the tubes towards the down-take where it flows downwards through the down-take before re-entering the tubes at their base. Whilst mechanical stirrers installed in the down-take can be used to improve circulation (stirred pans) it is preferable to ensure that the boiling process itself produces good circulation (natural circulation pans).
It was once believed that the driving force for circulation was the density difference between hot massecuite in the tubes and colder massecuite within the down-take. It has since been shown that the lower average density of boiling massecuite, i.e. a combination of massecuite and vapour bubbles, provides the circulating driving force when compared with the massecuite (without vapour bubbles) in the down-take.
Good circulation results when there is a good driving force and a low resistance to flow. One major decision in achieving this is the selection of tube diameter to improve the driving force whilst minimising the frictional drag. A tube diameter of approximately 100 mm is often chosen as the best compromise in this regard. The other major factor is the relative size and location of the down-take. The “circulation ratio” of a pan is a parameter often used to characterise this aspect of pan design. The “circulation ratio” is the ratio of the total cross-sectional area of all the tubes to the cross-sectional area of the down-take and a value of 2.5 is considered to be appropriate for circular batch pans with a centrally located cylindrical down-take. Larger circulation ratios imply a relatively small down-take and are known to result in poorer circulation.
Whilst the circulation ratio is effective in selecting the appropriate size of a single cylindrical down-take, it has been shown that it is not applicable in the same way for other designs of down-takes. A specific example is in “floating calandria” pans where the shape of the down-take is an annulus adjacent the pan wall. A number of poorly performing pans of this design have been subsequently modified to have conventional calandrias with central cylindrical down-takes. It is recognised that an annular down-take will have a proportionally greater wall area, and thus more drag, than a cylindrical down-take of the same cross-sectional area and that this can to some degree explain the poor performance of floating calandria pans. The extra drag of a non circular down-take can to some extent be taken into account by using the concept of a hydraulic diameter″ (defined as 4* the cross-sectional area divided by the wetted perimeter). The applicant's continuous pan design, as taught in U.S. Pat. No. 6,991,708, has a non-circular down-take adjacent the outside wall of the pan and is thus somewhat similar to a floating calandria. This pan achieves good circulation by using a smaller circulation ratio than appropriate for a conventional batch pan with a central cylindrical down-take.
A factor that does not appear to have been taken into account in pan design is the extra drag that will result from boiling taking place on the wall of the down-take that is shared with the calandria, i.e. where there is heating steam on the opposing side of the wall from the massecuite. This may even result in a portion of the massecuite that is closest to the wall flowing upwards in the down-take. It is likely that this effect will be greatest in pans with non-cylindrical down-takes, such as floating calandria pans or continuous pans with down-takes adjacent to the outside wall.
It is believed that the circulation within a pan will be improved if boiling within the down-take can be prevented by limiting or preventing heat transfer from the steam within the calandria to massecuite in the down-take.
It is therefore an object of the invention to provide a continuous vacuum pan that will, at least partially, overcome the above disadvantages.
It is also an object of the invention to provide a continuous vacuum pan which will be a useful alternative to existing vacuum pans.
It is a still further object of the invention to provide a continuous vacuum pan utilizing a calandria insulation arrangement that will reduce heat transfer between the calandria and massecuite in the down-take.